Sulfuric acid usually is produced by the so-called double absorption process as it is described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A25, pages 635 to 700. Sulfur dioxide (SO2) obtained as waste gas of metallurgical plants or by combustion of sulfur is converted to sulfur trioxide (SO3) in a multistage converter by means of a solid catalyst, e.g. with vanadium pentoxide as active component. The SO3 obtained is withdrawn after the contact stages of the converter and supplied to an intermediate absorber or after the last contact stage of the converter to a final absorber in which the gas containing SO3 is guided in counterflow to concentrated sulfuric acid and absorbed in the same.
The absorption of the SO3 in sulfuric acid is a strongly exothermal process, so that the acid is heated up and must be cooled again. At the same time, the heat of the acid can be utilized for steam generation and energy recovery. Due to the temperatures of distinctly >140° C., which exist during the sulfuric acid absorption, acid cooling so far has exclusively been effected in kettle-type boilers, wherein the hot acid flows through U-shaped tubes to be passed through a kettle filled with water as heat transport medium. The circulation here is based on the thermosiphon principle. The heated water is converted into low-pressure steam and rises due to the lower density. The steam can be utilized in the plant (cf. Ullmann's Encyclopedia of Industrial Chemistry, loc. cit. p. 662).
While such kettle-type boilers have a simple construction and therefore can be manufactured at low cost, large amounts of water are required for filling the kettle type boiler. In addition, problems can arise when leakages occur in the acid circuit. Since acid leaks from the tubes into the water tank, a large amount of a highly corrosive weak acid is obtained, whose temperature in addition greatly rises due to the hydration heat produced. The corrosion resistance of the steels used in the system greatly drops below a sulfuric acid concentration of 99.1 wt-% (steel 310SS) or 97.9 wt-% (steel 3033). There is a risk of damage of the tube bundle or even of the entire kettle-type boiler. In addition, the acid/water mixture can only be separated with a disproportionate effort, so that in practice the user mostly will have to completely empty one or both of the systems.